1. Field of the Invention
The present invention relates to a polymer blend for preparing liquid crystal alignment layer, more specifically, to a blend of cinnamate polymer and polyimide polymer, which are photo-reactive polymers, for preparing liquid crystal alignment layer having a high pretilt angle in photo-alignment, a process for preparing liquid crystal alignment layer by employing the said blend, a liquid crystal alignment layer prepared by the process, and a crystal cell prepared by employing the liquid crystal alignment layer.
2. Description of the Prior Art
Liquid crystal displays(xe2x80x9cLCDsxe2x80x9d) have been widely used in laptop computers and car navigation systems, since they are light and small compared to the existing cathode ray tube. Further, the recent increased demand for large monitors as well as the advantages of the lightness and thinness of liquid crystal displays drives the use of LCD to desktop monitors. Most LCDs are thin film transistor-liquid crystal displays (TFT-LCD), in which the light from a fluorescence lamp is incident to a liquid crystal panel by a reflection or dispersion device, and the incident ray is passed through or intercepted by the liquid crystal layer, of which birefringence is changed depending on the voltage controlled by the thin film transistor(TFT) to display an image.
The liquid crystal panel consists of two glass plates filled with twisted nematic liquid crystals between the plates: One glass plate upon which the light is incident, comprises a thin film transistor, pixel made of indium tin oxide (ITO) and a liquid crystal alignment layer, while the other glass plate is equipped with a color filter and coated with a liquid crystal alignment layer. A polarizer is attached to the outside of both of the two glass plates. The liquid crystal alignment layer with liquid crystals is aligned in a parallel and perpendicular manner against the surface of the plate, where the liquid crystals are mechanically contacted by a rubbing method using a soft cotton or nylon velvet to a plate coated with polymers. However, the liquid crystal alignment by the rubbing method is less satisfactory in the senses of a breakage of thin film transistor, a cross-track-short due to static charge build-up and an increase of inferior quality products caused by a dust.
To overcome these shortcomings of the rubbing method, several non-contact methods for aligning liquid crystals have been studied. Among the methods, a photo-alignment method that aligns liquid crystals by means of photo irradiation to prepare a liquid crystal alignment layer, employs polymers aligning liquid crystals by means of photo-dimerization, photo-isomerization and photodecomposition, respectively. However, the polymers have revealed defects that they require a long period of photoirradiation due to a slow photoreaction or bring a low pretilt angle so the polymers are not practically employed and manufactured into a liquid crystal alignment layer.
Under the circumstances, there are strong reasons for exploring and developing an alternative polymer which can overcome the said problems for preparing a liquid crystal alignment layer.
The present inventors have made an effort to develop a novel polymer to overcome the defects of the polymers employed for a photo-alignment method, and prepared a liquid crystal alignment layer employing a polymer blend comprising cinnamate polymer and polyimide polymer, and found that a liquid crystal cell prepared by employing the liquid crystal alignment layer has a high pretilt angle and an excellent thermal stability.
The first object of present invention is, therefore, to provide a polymer blend for preparing liquid crystal alignment layer, which comprises cinnamate polymer and polyimide polymer.
The second object of the invention is to provide a process for preparing liquid crystal alignment layer by employing the polymer.
The third object of the invention is to provide a liquid crystal alignment layer prepared by the process.
The fourth object of the invention is to provide a liquid crystal cell prepared by employing the liquid crystal alignment layer.
A polymer blend for preparing liquid crystal alignment layer of the present invention comprises 10 to 90% (w/w) of cinnamate polymer and 10 to 90% (w/w) of polyimide polymer: The cinnamate polymer includes polyvinylcinnamate, polyalkoxycinnamate, polyvinylfluorocinnamate, polyvinylalkoxyfluorocinnamate, and a mixture or a copolymer of said polymers, which has 10 to 500 kDa of molecular weight in average and 1 to 20 carbons, and the polyimide polymer includes poly(pyromellitic dianhydride-4,4xe2x80x2-oxydiamine), poly(pyromellitic dianhydride-2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane), poly(pyromellitic dianhydride-2,2-bis(4-aminophenoxyphenyl)propane), poly(1,2,3,4-cyclobutanetetracarboxylic acid-4,4xe2x80x2-oxydiamine), poly(1,2,3,4-cyclobutanetetracarboxylic acid-2,2-bis-[4-(4-aminophenoxy)phenyl]-hexafluoropropane), poly(1,2,3,4-cyclobutanetetracarboxylic acid-2,2-bis(4-aminophenoxyphenyl)-propane), poly(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride-4,4xe2x80x2-oxydiamine), poly(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride-2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane), poly(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride-2,2-bis(4-aminophenoxyphenyl)propane), poly(pyromelliticdianhydride-2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane)-4,4xe2x80x2-oxydiamine) or a copolymer of more than two of the said polymers, which has 10 to 300 kDa of molecular weight in average.
Meanwhile, the process for preparing liquid crystal alignment layer of the invention comprises the steps of: mixing cinnamate polymer and polyimide polymer in a ratio of 1:9 to 9:1 (w/w), and dissolving the nixed polymers in an organic solvent; spin coating the dissolved polymers onto a glass plate; heating the polymers to obtain an alignment layer; and, irradiating ultraviolet ray to the alignment layer to align liquid crystals in preferred direction Preferably, the organic solvent includes N-methyl-2-pyrrolidone, dimethylsulfoxide, dimethylacetate, metacresol, n-butylacetate and diethylether, and, the spin coating is performed at 1,000 rpm to 3,000 rpm for 10 seconds to 10 minutes and the heating is performed at the temperature of 100xc2x0 C. to 300xc2x0 C. for 10 seconds to 10 minutes. The ultraviolet is preferably irradiated by subjecting a polarized ray and/or a nonpolarized ray at an angle 0xc2x0 to 89xc2x0 with respect to a normal of surface of the layer.
In addition, a liquid crystal cell is prepared by filling with liquid crystals between two layers of the liquid crystal alignment layer prepared above.
Pretilt angle is determined by measuring the intensity of a laser beam after irradiating the laser beam to the direction of a normal with respect to the surface of liquid crystal cell, in the course of rotating the cell. In general, in the case of applying an electric field to operate a cell, liquid crystals filled in the cell stand vertical while the removal of electric field makes liquid crystals stand horizontal. Inasmuch as the standing direction is non-uniform, the cell comes to have a defect. Meanwhile, the higher the pretilt angle of liquid crystal alignment layer which makes up a liquid crystal cell is, the more the standing direction of liquid crystals is uniform. The liquid crystal cell prepared by employing the liquid crystal alignment layer of the invention shows an excellent pretilt angle compared to the conventional liquid crystal alignment layers.
On the other hand, an azimuthal anchoring energy was measured to determine the thermal stability of liquid crystal cell. The azimuthal anchoring energy determines how the liquid crystals are fixed strongly to the alignment layer and expressed as the thickness of Neel Wall observed in the liquid crystal cell. Generally, the less the energy values are, the worse the alignment of liquid crystals are. The thermal stability of liquid crystal cell is determined on the basis of the extent of lowering in an azimuthal anchoring energy in a heat-treated liquid crystal cell. Since the liquid crystal cell prepared by employing the liquid crystal alignment layer of the invention maintains a good liquid crystal alignment even under a high temperature, thermal stability is excellent. That is, the liquid crystal alignment layer has a high pretilt angle and an excellent thermal stability, which makes possible its wide application in the development of LCDs.
The present invention is further illustrated by the following examples, which should not be taken to limit the scope of the invention.